The present invention relates to the plating of aluminum or its alloys onto an electrically conductive substrate serving as the cathode in an electrolytic cell. More specifically, it relates to the formation of an inexpensive and effective electroplating solution which can be employed for electroplating from solution using an inert anode.
Scientists have known for a long time that aluminum cannot be electrodeposited successfully from aqueous solutions of its salts because electrolysis merely results in the decomposition of the water present in the electrolyte. This is true since the potential necessary for the deposition of aluminum is much in excess of that necessary for the liberation of hydrogen.
A number of processes have been reported in the literature in which aluminum is plated from a bath containing aluminum compounds in organic solutions. The processes disclosed are: U.S. Pat. No. 2,170,375 issued Aug. 22, 1939 to Mathers et al., U.S. Pat. No. 2,763,605 issued Sept. 18, 1956 to Miller et al., U.S. Pat. No. 3,355,368 issued Nov. 28, 1967 to Smith et al., U.S. Pat. No. 2,651,608 issued Sept. 8, 1953 to Brenner et al., U.S. Pat. No. 3,775,260 issued Apr. 27, 1971 to Capuano et al., as well as articles entitled "Electrolytic Precipitation of Aluminum from Solutions of Aluminum Bromide in Xylene" by A. Levinskene and L. SimanaVichyus, in Scientific Works of the Universities of the Lithuanian SSR Chemistry and Chemical Technology VII (1965); and "The System Benzene-Hydrogen Bromide-Aluminum Bromide" by Eley and King, J. Chem Soc., 2517 (1952). However, the processes cited in the above references have major disadvantages.
The Mathers work as discussed in U.S. Pat. No. 2,170,375 for example, does not result in efficient plating, and it found that, because of the concentrations and ratios of the various reagents employed by Mathers, that plating of aluminum, even at high voltages, is sporadic and, in many instances, non-existent. Capuano confirms this, in his discussion of the Mathers work, in U.S. Pat. No. 3,775,260. Further, Mathers uses preformed aluminum bromide which is very expensive, in scarce supply, and difficult to work with because it picks up water readily from the atmosphere. The water then reacts with the aluminum bromide to form aluminum oxide; thus aluminum is not available for the plating operation.
Another disadvantage of Mathers, is his use of an aluminum anode. If any oxide on the surface of the aluminum anodewere not to be removed evenly, spots of highly insulative aluminum oxide would remain on the anode, and regions on the cathode corresponding to these spots would be plated only to a relatively low degree. Also, metallic impurities in the aluminum anode may plate in preference to aluminum.
With regard to the relatively recent Capuano et al., U.S. Pat. No. 3,775,260, it is stated that a small amount of water is necessary in the solution for conducting purposes. The presence of water, however, leads to the copious evolution of HBr fumes during electrolysis, and the aluminum which plates on the substrate is a dull grey and somewhat brittle. Furthermore, the process requires the use of an expensive ethylbenzene which is consumed during electrolysis.
In general, processes cited above have not achieved commercial success because the electrolytic baths have normally been unstable; often contain inflammable or possibly explosive organic materials, and are usually toxic, complex, and relatively expensive to formulate. Since such solutions in general are not too conductive, it has been found necessary to employ either a volatile solvent such as ether or a compound which evolves copious quantities of a poisonous gas. Such baths are hazardous to work with. In addition, the plated aluminum is usually not high quality, being dull and brittle and not very adherent to the substrate.